Connecting boxes which are provided with a connecting socket which can be covered are increasingly required for electrical connections in the industrial world, for example for data lines, electrical cables and connections which are used in particular in a manufacturing environment, that is to say in factory or assembly halls, or else in the field of technical maintenance and repair, for example in workshops. In this case, the requirements are comparatively stringent with regard to protection of the connection located in the connecting socket against dust, items used during operation or cleaning agents, particularly in the state when a plug is not fitted, for which reason it is necessary for the plug-in socket cover to be sealed, for example, against oil and dirt, or else against cleaning agents as well as water splashes and water condensation. In this case, folding covers are frequently used because they are simple to handle, although they are relatively difficult to seal owing to the non-uniform contact forces acting on the seal. Thus, when the requirements relating to the sealing of the cover are stringent, conventional seals generally do not provide satisfactory results. Suitable seals which may be used for the described application also have to satisfy further preconditions, in addition to providing adequate sealing. For example, on the one hand, it is necessary to have the capability for the seal to be fitted and fixed easily. On the other hand, simple handling must be possible in the daily operating conditions, for example during replacement of the seal or during cleaning, and it must be possible to ensure that the sealing effect is produced easily and reproducibly.